These are NOAD's versions of the terms:

> **sincere** |sinˈsi(ə)r|
adjective ( -cerer , -cerest )
free from pretense or deceit; proceeding from genuine feelings : *they offer their sincere thanks to Paul.*
• (of a person) saying what they genuinely feel or believe; not dishonest or hypocritical.


> **honest** |ˈänist|
*adjective*
free of deceit and untruthfulness; sincere : *I haven't been totally honest with you.*
• morally correct or virtuous : *I did the only right and honest thing.*
• [ attrib. ] fairly earned, esp. through hard work : *struggling to make an honest living.*
• (of an action) blameless or well intentioned even if unsuccessful or misguided : *he'd made an honest mistake*.
• [ attrib. ] simple, unpretentious, and unsophisticated : *good honest food with no gimmicks.*

As you see, the meanings of the two words intersect, but there are real points of difference in some of the senses. There are cases where you would not substitute one for the other. *Sincere* is hardly going to be understood to mean "simple, unpretentious, and unsophisticated" for example. And *honest* has more to do with veracity than feelings.